There are several known types of MIMO techniques, the general scheme of which is illustrated in FIG. 1, for a sent signal x and a received signal {circumflex over (x)} with nt sending antennas, nr receiving antennas and a MIMO channel matrix denoted as Hnt×nr.
The initial goal of MIMO techniques is to exploit the spatial and temporal diversity (or frequency diversity) of multi-antenna systems and to improve their capacity.
However, the error-correcting function, also called the channel-decoding function, has been taken into account only recently in the building of MIMO space-time codes, as is described in the document “M. S. Hassan, K. Amis, “Turbo-Product Codes Decoding for Concatenated Space-Time Error-Correcting Codes”, 20th International Symposium PIMRC '2009, pp. 1049-1053, 13-16 Sep. 2009”.
Certain of these MIMO techniques, known as “coherent” techniques require perfect knowledge of the channel when sending or receiving, and other techniques, called “non-coherent” techniques do not require any knowledge of the channel whether at sending or at receiving.
These coherent MIMO techniques include especially two families: the STBC (space-time block codes) techniques and the STTC (space-time trellis codes) techniques.
The existing non-coherent MIMO techniques implement block encoding adapted to the above-mentioned coherent STBC or STTC techniques. These non-coherent MIMO techniques correspond actually to an extension, to the MIMO case, of the differential technique used for SISO (single-input single-output) differential systems: this is the classic case with a single antenna for sending and receiving.
The basic principles of the differential codes is the following: the symbol transmitted at the instant (t) is equal to the symbol transmitted at the instant (t−1) multiplied by a complex matrix Vt or a simple complex carrier of the information. The information to be transmitted is therefore to be encoded by the transition from one symbol to another and not by the value or state of each symbol.
The basic hypothesis for this differential technique is that the channel is assumed to be constant for a long data block (at least several hundreds of data pieces) to be transmitted.
The simplest solution for extending these differential codes to the MIMO case is to use orthogonal space-time codes because they enable the generation of perfectly orthogonal virtual SISO channels in the MIMO channel and thus enable the performance of differential encoding on each virtual channel. There are several examples of this in the literature, some of them especially present in the document by H. Bölckei and M. Borgmann, “Codes Design for Non-Coherent MIMO-OFDM Systems” Allerton Conference 2002, pp. 237-246.
These non-coherent MIMO differential techniques however do not implement any error correcting and therefore do not have any encoding gain. Their sole goal is to enable the detection of the data transmitted without knowledge of the propagation channel.
A first drawback of these techniques lies in a 3 dB loss of performance as compared with transmissions made with knowledge of the propagation channel, at least in part because of the absence of encoding gain due to the absence of error-correcting encoding.
One solution to the question of obtaining encoding gain for these differential techniques is to combine them with an error-correcting code in the form of two disjointed functions. The performance is then improved but the 3 dB loss at the differential detection level cannot be compensated for. To try to overcome this drawback, certain studies have been conducted in order to set up an iterative reception as presented for example in the document B. L. Hughes, “Differential Space-Time Modulation”, IEEE transactions on Information Theory, Vol. 46, No. 7, pages 2567-2578, November 2000. This iterative reception makes it possible to communicate the two detection and error-correcting functions successively and iteratively by exchange of intrinsic information between the two functions, in order to enable the reduction only in part of this 3 dB loss resulting from the differential detection.
However, the main drawback of these differential techniques lies in the need to assume that the channel is constant during a long block of data elements to be transmitted, which is the basic assumption of these techniques. This assumption is indeed highly constraining and unrealistic, especially for the radio-electrical channels used in practice for transmissions to and from the mobile terminals.
There is therefore a need for a new technique for transmitting non-coherent MIMO data by which it is possible to overcome the constraint which assumes that the channel is constant for a long block of data to be transmitted and offers better performance in terms of transmission bit rate and/or quality of service for the clients.